1. Field of the Invention
The present teachings generally relate to rectilinear measuring systems and articulated arm coordinate measuring machines and more particularly to a system for automated measuring arm positioning.
2. Description of the Related Art
Rectilinear measuring systems, also referred to as coordinate measuring machines (CMM's) and articulated arm measuring machines including portable coordinate measuring machines (PCMM's) have been described for generating geometry information from various objects and areas. In general, these instruments capture the structural characteristics of an object for use in electronic rendering and duplication. One example of a conventional apparatus used for coordinate data acquisition comprises a support and a moveable measuring arm made up of hinged segments to which a contact-sensitive probe or remote scanning device is attached. Geometry information or three-dimensional coordinate data characterizing the shape, features, and size of the object may be acquired by tracing or scanning along the object's surface and contours. Probe or scanning device movement is typically tracked relative to a reference coordinate system resulting in a collection of data points and information that may be used to develop an accurate electronic rendering of the object. In conventional implementations, the acquired geometry information is processed by a computer capable of making use of the information to model the surface contours and dimensions of the object.
One limitation of many conventional instruments is that they are generally sensitive to external physical perturbations including vibrations and fluctuations in temperature which may degrade the accuracy of coordinate acquisition. For example, it may be necessary to perform coordinate calibration processes several times in a particular environment where the ambient temperature changes even a few degrees to compensate for thermal expansion and contraction of joints and components in an instrument. In articulated measuring arms, the components that make up the arm segments and hinged portions of the measuring arm are particularly susceptible to localized thermal effects affecting the performance of the instrument and can impart undesirable distortions and inaccuracies in coordinate acquisition. Additionally, imperfections in hinge, actuator, and motor design can result in a certain degree of variability or “slop” in measuring arm movement further affecting the overall instrument accuracy.
Another problem with existing designs is that inadvertent jarring of the instrument by an operator or other vibrations may result in degradation of coordination acquisition performance. Consequently, conventional instruments must be treated as highly-sensitive pieces of equipment and are generally set up in a controlled environment to insure maximum accuracy and reliability. Despite these considerations it is not uncommon for an instrument to require realignment or recalibration during routine operation thus increasing the time required to obtain a complete coordinate set for a selected object.
For the aforementioned reasons of environmental sensitivity as well as the generally large overall size, weight, and complexity of the instrument itself, conventional instruments are also not well suited for adaptation to portable platforms which include motor-assisted measuring arm articulation or robotic control. Development of a powered means for assisting in measuring arm positioning presents a number of design considerations that should be addressed to insure sufficient reliability and precision in coordination acquisition. These factors include evaluating how motors and actuators should be positioned about the measuring arm to reduce or offset thermal effects as well as considering how these components might best be positioned to increase overall stability and reduce vibrations affecting the instrument.
From the foregoing it will be appreciated that there is a need for an improved means of vibration damping and thermal compensation in coordinate acquisition instruments including CMMs and PCMMs. Additionally, there is a need for an instrument platform capable of motor-assisted or robotically controllable movement that is relatively easy to calibrate and retains a high degree of accuracy and sensitivity. Such an instrument would be of substantial benefit in a number of different applications and provide increased flexibility over conventional designs.